Device for producing a reference frequency signal

ABSTRACT

The invention relates to a device ( 1 ) for producing a reference frequency signal (Se) from the response of an atomic resonator (R) to a pumping signal (Fat) transmitted thereto, comprising: a first oscillator for production of a first signal (S 1 ) of frequency (F 0 ) as a function of the response signal (E 1 ) from the resonator (R); a second oscillator ( 13 ) for production of a second signal (S 2 ) at a frequency (NF 0 ) which is equal to a whole multiple of that of the first signal (S 1 ). According to the invention, the second oscillator is selected to produce a second signal (S 2 ) close in frequency to the frequency of resonance for the resonator (R).

[0001] The present invention relates to apparatus for supplying astandard frequency signal on the basis of the response of an atomicresonator to a driver signal which is applied thereto.

BACKGROUND OF THE INVENTION

[0002] Atomic resonators, e.g. cesium resonators, are used because ofthe precision of their atomic transition defining a standard frequencythat is exact and very stable. In apparatus designed for this purpose, adriver signal is synthesized at the resonant frequency of the resonator,it is delivered in modulated form to the resonator, and the responsesignal of the resonator is picked up and demodulated, with the frequencyof a first oscillator then being servo-controlled on the basis of thedemodulated signal. Such apparatus enables a standard signal to begenerated having frequency which is very exact and very stable, towithin about 10⁻¹² and 10⁻¹⁴, respectively.

[0003] Such apparatuses are required, for example, in spaceapplications, for navigation by satellite.

[0004] U.S. Pat. No. 5,148,122 describes apparatus of the kind describedabove, for a cesium resonator comprising:

[0005] a first oscillator at 10 megahertz (MHz) suitable for producing afirst signal at a frequency which is a function of the response signalfrom the resonator;

[0006] a second oscillator producing a second signal at 650 MHz as afunction of a control signal;

[0007] means for servo-controlling the phase of the second signal on thefirst signal, producing the control signal; and

[0008] means for generating the reference signal and the driver signaldownstream from the second oscillator, which means include frequencydividers and multipliers.

[0009] That apparatus turns out to present drawbacks. It requires alarge number of component elements, thereby making it complex, heavy,and voluminous. It consumes a large quantity of energy and it isexpensive.

[0010] The document entitled “A new RF architecture for cesium frequencystandards”, published in pp. 134 to 142 of 1992 IEEE Frequency ControlSymposium, discloses similar apparatus further comprising a thirdoscillator downstream from the multiplier and divider circuit andproducing the driver signal at the resonant frequency.

[0011] Of even greater complexity, that apparatus additionally createsdisturbance by electromagnetic radiation, and the atomic resonator issensitive thereto, because an oscillator is being used at its resonantfrequency. Overcoming such disturbance requires shielding, which isexpensive, heavy, and bulky.

[0012] The document entitled “Cs frequency synthesis: a new approach”,published on pp. 615 to 619 of Volume 2 of 1999 Joint Meeting EFTF—IEEEIFCS, describes a cesium resonator frequency synthesizer havingoscillators at 5 MHz, 100 MHz, and 6.4 gigahertz (GHz). An analogregenerator divider responds to the oscillator at 6.4 GHz to generatesignals at 3.2 GHz and 9.6 GHz. A signal at 9.192 GHz for the cesiumresonator is generated after mixing a signal at 407.368 MHz with thesignal at 9.6 GHz.

[0013] That synthesizer likewise comprises a large number of circuitsincluding an analog regenerator divider which must be made specially forthe synthesizer.

OBJECTS AND SUMMARY OF THE INVENTION

[0014] The invention seeks to provide apparatus mitigating the drawbacksof the prior art, that is simple in structure, comprising onlyconventional commercially-available circuits, and only a relativelysmall number thereof.

[0015] To this end, the invention provides apparatus for delivering astandard frequency signal on the basis of the response of an atomicresonator to a driver signal which is applied thereto, the apparatuscomprising:

[0016] a first oscillator suitable for producing a first signal offrequency that is a function of the response signal of the resonator;

[0017] a second oscillator suitable for producing a second signal offrequency equal to an integer multiple of the frequency of the firstsignal, as a function of a control signal;

[0018] phase servo-control means for servo-controlling the phase of thesecond signal on the first signal, thereby producing the control signal;and

[0019] means for generating the driver signal from the second signal;

[0020] the apparatus further comprising means for directly delivering atleast one standard signal from the first signal and connected downstreamfrom the first oscillator and upstream from the second oscillator, andthe second oscillator is selected in such a manner as to produce asecond signal of frequency equal to a value lying in a range of 1 GHz oneither side of the resonant frequency of the resonator.

[0021] The invention makes it possible to omit the frequency multiplierand divider circuits downstream from the second oscillator forgenerating the standard signal and the driver signal.

[0022] According to a characteristic of the invention, the secondoscillator is selected to produce a second signal of frequencysubstantially equal to one of the twenty integer multiples of the firstsignal that are closest to the resonant frequency of the resonator.

[0023] According to other characteristics of the invention, enabling thestructure of the apparatus to be further simplified:

[0024] the second oscillator is selected in such a manner as to producea second signal of frequency lying in a range of 50 MHz on either sideof the resonant frequency of the resonator; and

[0025] the second oscillator is selected in such a manner as to producea second signal of frequency substantially equal to one of the twointeger multiples of the frequency of the first signal that are closestto the resonant frequency of the resonator.

[0026] According to a characteristic that enables good phase coherenceto be obtained between the first and second signals, the phaseservo-control means comprise a logical frequency divider circuit fordividing the second signal by the integer multiple, a phase comparatorcircuit for comparing the phase of the first signal with the phase atthe output of the circuit for dividing the second signal, and a circuitfor generating a control signal for the second oscillator from thesignal output by the comparator circuit.

[0027] According to another characteristic of the invention, making itpossible both to dispense with multipliers for generating the driversignal and to avoid the second oscillator disturbing the resonator, thesecond oscillator is selected in such a manner as to produce a secondsignal at a frequency that is offset relative to the resonant frequencyof the resonator, the generator means comprising, downstream from thesecond oscillator, a circuit suitable for adding and/or subtracting anoffset signal to or from the second signal so as to deliver the driversignal, and a circuit for delivering the offset signal. This makes itpossible to adjust the power of the driver signal delivered to theresonator via the frequency adder and/or subtracter circuit to deliverjust enough power to the resonator. In addition, any fluctuation in thepower of the second signal does not significantly modify the power ofthe driver signal, which can be controlled proportionally by theoffsetting circuit. This avoids any problems of aging in the secondoscillator.

[0028] In an embodiment, the adder and/or subtracter circuit isconstituted by a mixer circuit, e.g. a single-sideband circuit.

[0029] In an embodiment, the circuit for delivering the offset signalcomprises a frequency synthesizer synchronized on the first signal.

[0030] According to a characteristic of the invention, the means forgenerating the driver signal comprise, downstream from the secondoscillator, only circuits for adding and/or subtracting frequency to orfrom the second signal.

[0031] In an embodiment of the invention, suitable for generatingstandard frequency signals from the response of a cesium atomicresonator, the second oscillator is suitable for producing a secondsignal at 9200 MHz, and the first oscillator is suitable for deliveringa first signal at 10 MHz.

[0032] According to a characteristic of the invention, the apparatuscomprises, as oscillators, only the first and second oscillators.

BRIEF DESCRIPTION OF THE DRAWING

[0033] The invention will be better understood in the light of thefollowing description made with reference to the drawing given purely byway of example and constituting a diagram of apparatus in accordancewith the invention for supplying a standard frequency.

MORE DETAILED DESCRIPTION

[0034] The apparatus 1 for supplying a standard frequency is describedwith reference to a cesium resonator R. Naturally, the apparatus couldequally well be used with any other atomic resonator, for example arubidium resonator.

[0035] The apparatus 1 for supplying a standard frequency comprises afirst oscillator 2 outputting a first signal S1 of determined frequencyF0 adjusted by an adjustment signal received on an adjustment input E1.The first oscillator 2 is of the voltage controlled crystal oscillator(VCXO) type, for example, and operates at 10 MHz, for example. Theadjustment input E1 is connected to a monitor circuit C which deliversan adjustment signal as a function of the response signal Sr from theresonator R. For a description of the operation of the resonator R andthe monitor circuit C, reference can be made to U.S. Pat. No. 5,148,122where these elements are designated respectively by references 11 and 26and where the corresponding description is given from column 3, line 47to column 4, line 51.

[0036] The output from the second oscillator 2 is connected to a circuit3 for distributing the first signal S1 and for supplying the standardsignal Se. Naturally, a plurality of standard signals can be supplied bythe circuit 3. Standard signals at frequencies which are submultiples ofthe frequency of the standard signal Se at 10 MHz may also be suppliedby appropriate circuits situated downstream from the signal Se and thedelivery circuit 3. The circuit 3 comprises a circuit 3 a for sharingthe power delivered by the first signal S1, with each of its outputsconnected to a respective broadband isolating amplifier 3 b of low gainoutputting a respective signal at frequency F0. One or more isolatoramplifiers 3 b output the standard signal Se at the same frequency F0.An output 4 of another isolation amplifier 3 b of the distribution anddelivery circuit 3 is connected to a first input 5 of a phase comparatorwhose second input 7 is connected to the output 8 of a broadband logicalfrequency divider 9. The output 10 from the phase comparator isconnected to the input 11 of a circuit 12 for generating the controlsignal of a second oscillator 13 operating at microwave frequency. Thefrequency divider 9 can be implemented as a plurality of divider stagesin cascade.

[0037] The output 14 from the control signal generator circuit 12 isconnected to a control input E2 of the second oscillator 13 whose output15 delivers a second signal S2 of determined frequency as a function ofthe signal present on its control input E2. The second signal S2 issituated at a frequency that is within 1 GHz above or below the resonantfrequency of the resonator R. The frequency of the second signal is aninteger multiple N of the frequency F0 of the first signal S1. Thesecond signal S2 is at a frequency close to and different from theresonant frequency of the resonator R and is at a frequency equal to oneof the twenty integer multiples of the frequency of the first signal S1that are closest to the resonant frequency, and for example one of thetwo integer multiples of the frequency of the first signal S1 that areclosest to the resonant frequency. In the embodiment shown, thefrequency of the second signal S2 produced by the second oscillator is9200 MHz for a cesium resonator having a resonant frequency of9,192,631,700 hertz (Hz).

[0038] The output 15 of the second oscillator 13 is applied to a coupler16 connected via a first output 16 a to the input 9 a of the frequencydivider 9 whose division ratio is equal to the above-mentioned integermultiple N. The signal present on the output 8 of the divider 9possesses a fundamental frequency equal to the frequency F0 of the firstsignal. The control signal generator circuit 12, the divider 9, and thecomparator 6 are such as to form a circuit for locking the phase of thesecond signal S2 on the first signal S1.

[0039] A second output 16 b of the coupler 16 is connected to a firstinput 17 b of a frequency mixer circuit 17 whose second input 18receives a frequency offset signal D and whose output delivers a signalFat for driving the atomic resonator R and having its resonantfrequency. The mixer circuit 17 adds and/or subtracts the frequency ofthe offset signal D to or from the second signal S2. The offset signal Dis frequency modulated by a signal at very low frequency, for exampleseveral tens of hertz, and is produced at the output 19 of a deliverycircuit 20. In the example above, the circuit 20 delivers the offsetsignal D at 7.368230 MHz. The delivery circuit 20 is formed, forexample, by a low frequency direct digital synthesizer (DDS) typecircuit. The delivery circuit 20 is synchronized via its synchronizationinput 21 which is connected to an output 22 of the distribution circuit3.

[0040] In an embodiment enabling a less expensive circuit to be used,the mixer circuit 17 delivers two sidebands and outputs a frequency sumsignal and a frequency difference signal based on the second signal S2and the offset signal D, and in the above example these are atfrequencies of 9,207.368230 MHz and 9,192.631770 MHz respectively.

[0041] In an embodiment avoiding the interference effects produced bythe output signal at 9,207.368230 MHz so as to obtain better frequencyexactness and stability, the mixer circuit 17 is a single-sidebandcircuit selected to conserve at its output the driver signal Fat at theresonant frequency while attenuating the signal at the frequency of9,207.368230 MHz.

[0042] The apparatus 1 also comprises an electricity power supply 23 forits various components. The apparatus 1 may be switched on directly byactuating a switch 24 provided on the power supply, without requiringadjustment.

[0043] The first and second oscillators 2 and 13 respectively can beselected in a higher performance version to obtain frequency exactnessand stability of about 10⁻¹² and 10⁻¹⁴ respectively, or in a lessexpensive lower performance version to obtain exactness and stability ofabout 10⁻¹¹ and 10⁻¹³, respectively.

What is claimed is:
 1. Apparatus for delivering a standard frequencysignal on the basis of the response of an atomic resonator to a driversignal which is applied thereto, the apparatus comprising: a firstoscillator suitable for producing a first signal of frequency that is afunction of the response signal of the resonator; a second oscillatorsuitable for producing a second signal of frequency equal to an integermultiple of the frequency of the first signal, as a function of acontrol signal; phase servo-control means for servo-controlling thephase of the second signal on the first signal, thereby producing thecontrol signal; and means for generating the driver signal from thesecond signal; the apparatus further comprising means for directlydelivering at least one standard signal from the first signal andconnected downstream from the first oscillator and upstream from thesecond oscillator, and the second oscillator is selected in such amanner as to produce a second signal of frequency equal to a value lyingin a range of 1 GHz on either side of the resonant frequency of theresonator.
 2. Apparatus according to claim 1, wherein the secondoscillator is selected to produce a second signal of frequencysubstantially equal to one of the twenty integer multiples of the firstsignal that are closest to the resonant frequency of the resonator. 3.Apparatus according to claim 1, wherein the second oscillator isselected in such a manner as to produce a second signal of frequencylying in a range of 50 MHz on either side of the resonant frequency ofthe resonator.
 4. Apparatus according to claim 1, wherein the secondoscillator is selected in such a manner as to produce a second signal offrequency substantially equal to one of the two integer multiples of thefrequency of the first signal that are closest to the resonant frequencyof the resonator.
 5. Apparatus according to claim 1, wherein the phaseservo-control means comprise a logical frequency divider circuit fordividing the second signal by the integer multiple, a phase comparatorcircuit for comparing the phase of the first signal with the phase atthe output of the circuit for dividing the second signal, and a circuitfor generating a control signal for the second oscillator from thesignal output by the comparator circuit.
 6. Apparatus according to claim1, wherein the second oscillator is selected in such a manner as toproduce a second signal at a frequency that is offset relative to theresonant frequency of the resonator, the generator means comprising,downstream from the second oscillator, a circuit suitable for addingand/or subtracting an offset signal to or from the second signal so asto deliver the driver signal, and a circuit for delivering the offsetsignal.
 7. Apparatus according to claim 6, wherein the adder and/orsubtracter circuit is constituted by a mixer circuit, e.g. a single sideband circuit.
 8. Apparatus according to claim 6, wherein the circuit fordelivering the offset signal comprises a frequency synthesizersynchronized on the first signal.
 9. Apparatus according to claim 6,wherein the means for generating the driver signal comprise, downstreamfrom the second oscillator, only circuits for adding and/or subtractingfrequency to or from the second signal.
 10. Apparatus for generatingstandard frequency signals from the response of a cesium atomicresonator according to claim 1, wherein the second oscillator issuitable for producing a second signal at 9200 MHz, and the firstoscillator is suitable for delivering a first signal at 10 MHz. 11.Apparatus according to claim 1, comprising, as oscillators, only thefirst and second oscillators.